It is known that the presence of mutans streptococci in the oral cavity of a human is closely associated with a development of dental caries. Mutans streptococci can attach to a surface of a tooth via insoluble glucan produced by mutans streptococci, and can grow by metabolizing sucrose to produce lactic acid in an acid environment. Lactobacillus, which can produce lactic acid and grow under a condition of an acid environment, is also considered a bacterial cause of dental caries, although Lactobacillus do not have an ability to attach to a surface of a tooth (non-patent reference 1, non-patent reference 2, and non-patent reference 3).
In a report of the non-patent references 1 to 3, the number of lactobacillus was measured by means of the culture method. It was demonstrated that a patient with high numbers of lactobacillus determined by the culture method has at high risk of a development of dental caries. Therefore, measurement of the number of lactobacillus is important for a prediction of the risk of development of dental caries. However, the conventional culture method takes two or more days to obtain a measurement result, and a procedure of the culture method is carried out by hand.
As a measuring method of the number of lactobacillus other than the culture method, a real-time PCR assay has been developed by designing primers having base sequences common among all kinds of lactobacillus, to thereby measure the number of lactobacillus in saliva or bacterial plaque, in a short time (non-patent reference 4). These primers are designed on the basis of common base sequence region in the 16S ribosomal RNA gene of genus Lactobacillus, including nine Lactobacillus species. However, the primers include a base sequences part which exhibits high homology to base sequences of oral bacteria other than lactobacillus, and so, it is considered that specificity of the primers is low. Further, it is not clear whether or not the number of lactobacillus obtained by the reported real-time PCR assay is correlated to that obtained by the culture method.
Therefore, the culture method is most appropriate for a predicting accurately the risk of development of dental caries by measuring the number of lactobacillus in an oral cavity. However, the culture method is very time-consuming, and thus is not efficient. Therefore, the culture method requires considerable labor when many samples are tested in, for example, a medical laboratory.
[Non-Patent Reference No. 1] The Journal of Clinical Pediatric Dentistry, U.S.A, 2005, vol. 29, p. 329-33
[Non-Patent Reference No. 2] Community Dentistry and Oral Epidemiology, Denmark, 1981, vol. 9, p. 182-90
[Non-Patent Reference No. 3] Journal of Oral Science, Japan, 2006, vol. 48, p. 245-51
[Non-Patent Reference No. 4] Journal of Clinical Microbiology, U.S.A, 2004, vol. 42, p. 3128-36